1. Field of the Invention
The present invention relates to a fixing device for electrophotographic apparatus, which is used to fix an unfixed toner image on a copy sheet by fusion with pressure.
2. Description of the Prior Art
Many conventional fixing devices for electrophotographic apparatus employ a so-called heating roller method for fixing a toner image. In this method, a heat source is provided inside a pair of rollers coated with a nonsticky material, the rollers are rotated under a suitable pressure therebetween, and a copy sheet onto which a toner image has transferred is passed through the pair of rollers for fixing the transferred toner image.
The recent technology has developed another method wherein a belt is wound around a pair of rollers and a third roller, and a transferred toner image on a copy sheet is then pressed against the surface of the belt with pressure for fixing. Examples of the prior art methods are as follows.
(a) Japanese Laid-open Patent Application No. 4-50883 discloses a fixing device as shown in FIG. 5, in which a fixing belt c with high thermal conductivity is stretched between and around a heat resisting fixing roller a having an elasticity and a heating support roller b, a pressing roller d is so arranged as to press the fixing roller a upwards through the fixing belt c, and a support member e carrying a transferred unfixed image e' is pressed by the fixing belt c and the pressing roller d for fixing image. This arrangement can prevent the heat resisting and elastic layer on the fixing roller a from peeling off because of the heat of a heater f. PA1 (b) Japanese Laid-open Patent Application No. 4-273274 discloses another apparatus as shown in FIG. 6, in which a fixing belt c.sub.1 is stretched around a heating fixing roller a.sub.1 and a support roller b.sub.1, a heating pressing roller d.sub.1 is arranged to press the fixing belt c.sub.1 against the heating fixing roller a.sub.1 from the lower side thereof, an image e' on a support member e is fixed between the fixing belt c.sub.1 and the pressing roller d.sub.1, the support member e is traveled by the fixing belt c.sub.1 while cooled at a guide g by a cooling device h and, after the cooling off, the image carrying support member e is separated from the fixing belt c.sub.1. This arrangement permits fixation and separation with no offset transfer without using a release agent such as a silicone oil. PA1 (c) Japanese Laid-open Patent Application No. 5-11651 discloses still another device as shown in FIG. 7, which is composed of a fixing roller a, a support roller b.sub.2, a fixing belt c.sub.2, a pressing roller d.sub.2 and a heater f for heating the fixing belt c.sub.2. This device further has a conveyer i for conveying an unfixed image carrying support member e arranged in parallel with and underneath the fixing belt c.sub.2. In this arrangement, a toner which forms an unfixed image e' is preliminarily heated prior to the fixation. This arrangement lowers the fixation temperature and makes the fixing device simple, compact and lower in cost. PA1 (d) Japanese Laid-open Utility Model Application No. 4-124267 discloses a device in which a main body of a fixing belt consists of a thin metal member. The main body of the belt is smoothly risen or cooled its temperature because of its high thermal conductivity. Thus, a high quality of image can be achieved by the belt type fixing even in the high-speed fixation. PA1 a fixing roller having an elastic surface; PA1 a heating roller for supplying a heat; PA1 an endless fixing belt consists of a metal body and a release thin film layer formed on a surface thereof, said fixing belt being stretched between the fixing roller and the heating roller, said metal body having a heat capacity per cm.sup.2 within the range of 0.001 to 0.02 cal/.degree.C.; PA1 a pressing roller for pressing said fixing roller through the fixing belt, said pressing roller forming a nip portion together with the fixing belt therebetween; and PA1 a guide plate disposed between the heating roller and the nip portion for guiding an unfixed image carrying support member to the nip portion, said guide plate forming substantially a linear heating path together with said fixing belt therebetween.
In the conventional device (a), nothing is disclosed as to the technical concept of heating preliminarily the unfixed image carrying support member e with use of the heated fixing belt c. In the conventional device (c), there is disclosed the preheating by the fixing belt c.sub.2, however, this prior art specifically concerns to the use of thermal dissociation type capsule toner for fixing the transferred image at low-temperature. Accordingly, the device (c) teaches nothing about the prevention of an offset transfer which may be caused by the use of high-temperature fixation toner.
Further, the conventional device (b) primarily concerns the forced cooling of fixation belt c.sub.1 being moved together with the image carrying support member e after fixing. The conventional device (d) concerns self cooling of the fixing belt through its structure while the fixing belt being traveled.